The auto contrast optimization is one of the widely used techniques for improving display qualities of panel display devices such as liquid crystal display devices. For example, contrast enhancement of a dark image under a situation that the brightness of a backlight is desired to be reduced effectively suppresses deterioration of the image quality with a reduced power consumption of the liquid crystal display device. In one approach, the contrast enhancement may be achieved by performing a correction calculation on image data (which indicate grayscale levels of each subpixel of each pixel). Japanese Patent Gazette No. 4,198,720 B discloses a technique for achieving a contrast enhancement, for example.
In general, currently-used auto contrast enhancement is achieved by analyzing image data of the entire image and performing a common correction calculation for all the pixels on the basis of the analysis; however, such auto contrast enhancement may cause a problem that, when a strong contrast enhancement is performed, the number of representable grayscale levels is reduced in dark and/or bright regions of images. A strong contrast enhancement potentially causes so-called “blocked up shadows” (that is, a phenomenon in which an image element originally to be displayed with a grayscale representation is undesirably displayed as a black region with a substantially-constant grayscale level) in a dark region in an image, and also potentially causes so-called “clipped white” in a bright region in an image.
Japanese Patent Application Publication No. 2004-23522 A discloses a contrast correction circuit which can reproduce an image with a high contrast regardless of the contents of the image. The disclosed contrast correction circuit is configured to calculate luminance frequency distribution information for each of a plurality of areas defined by dividing the display screen and to control the input-output characteristics of a grayscale level correction section on the basis of the frequency distribution information.
The contrast correction circuit disclosed in Japanese Patent Application Publication No. 2004-23522 A, however, suffers from a problem of block noise, because the contrast correction is individually performed for each area.